A demon is formed by what the summoner and the summoned think they should look like. If the summoner wanted to increase the varieties of demon, so that their would be demons that would appear less traditional perhaps, it would allow for more infiltration. Their are probably many people who are uncertain of the next life who might jump at the opportunity to have a new life if they had died or simply in a coma where they can’t reconnect with their body.
Also why do the Astlanians assume that the demons looked as they did on other planes and not question what could have naturally made something like a demon. Also since demons have mana bodies could they modify them by permanently incorporating other elements to make their bodies more dense, and capable of sustaining itself with less mana for easier infiltration of low mana realities as the current method is either through wet dreams, or through artificial bodies that are constructed that possesed. or through a naturally formed possesed body that may or may not still have a preexisting conscience within.